Storage batteries, especially of the nickel-cadmium type, are widely used as power sources for portable tools and appliances even where power from public utilities may be readily accessible. Batteries of relatively small capacity are used in such devices so that battery size and weight will not detract from the convenience of use and portability of the device in which the battery is installed. The small capacity of such batteries requires that they be frequently recharged, often after each use of the device in which they are installed, so that the device will be able to deliver full service at any needed time.
It is normal commercial practice, therefore, to include a battery charger with each storage battery operated device sold. Such chargers are of the simplest possible circuit design intended to receive input power only from a source of specified characteristics and to deliver charging current only to the battery contained in the device with which it is sold. The lack of universality of such chargers impairs the convenience of battery operated devices. If the device is to be used in an area where the utility power differs from that of the area in which the device was originally designed or power differs from that of the area in which the device was originally sold, it is necessary to replace the charger. If the charger designed for use with a particular device is misplaced or damaged, often a charger designed for another device cannot be substituted for the original charger.
A variety of battery charger circuits have been developed in response to these problems. Chargers of the type using a blocking oscillator as a current converter are of interest herein. In this type charger the charging current supplied to the battery is, in general, dependent upon a threshold set by a sensing part of the circuitry. This threshold is independent of the supply voltage in large part since it is designed to sense current. Therefore, this type of charger can be designed to accept input power from sources having a wide range of voltages while still delivering substantially constant charging current to the battery.